In optical transmission systems, optical phase modulation schemes that realize large capacities without deterioration at the time of transmission have recently gained attention. In particular, standardization by the Optical Internetworking Forum (OIF) is advancing for the scheme of Dual Polarization Differential Quadrature Phase Shift Keying (DP-QPSK), and this scheme will become a major scheme in the future.
A DP-QPSK optical demodulation circuit splits the polarization of a polarization-multiplexed optical phase-modulated signal (Sig) by a polarization beam splitter (PBS); causes, in a 90-degree hybrid circuit, the signal light of each polarization to interfere with local light (Lo) of a substantially equivalent optical frequency and that has not been modulated; and demodulates phase signals by differential reception into intensity signals. The 90-degree hybrid circuit sets the phase angle of the delay line to 90 degrees; a coupler is disposed respectively for an input optical phase modulated signal (signal light) and local light; and the phase rotation angle of the couplers is 90 degrees. Consequently, the amplitude of the local light is made larger and the amplitude of the signal light can be increased. Configuration of this 90-degree hybrid circuit requires accurate determination of the phase relationship of the light and the use of a planar optical waveguide (planar light-wave circuit (PLC)) that can be fabricated with precision is suitable.
To reduce device size and device cost under a DP-QPSK scheme, on the PLC, integration of a variable optical attenuator (VOA) that adjusts the attenuation of the signal light is demanded.
In the DP-QPSK scheme, integration of VOA on the PLC is demanded to reduce device size and device cost. Typically, the VOA is mounted before the polarization beam splitter (PBS). In this case, the problem that arises is a difference in attenuation consequent to polarization dependent loss (PDL) of the signal light split by the polarization beam splitter. The signal light transmitted by the polarization beam splitter is split into a TE wave and a TM wave, whose polarized light differs by 90 degrees. Since the refractive index in the waveguide differs, for light waves of polarized light that differ, a difference in the amount that each is attenuated arises. An example of performing optical attenuation by a VOA on input light that has been split by a polarization beam splitter (PBS) and control of PDL has been disclosed (for example, refer to Japanese Laid-open Patent Publication No. 2011-197700).
In a configuration where input light that has been split by a polarization beam splitter (PBS) and thereafter, is optically attenuated by a VOA, one component of the input light that has been transmitted through a circulator is TE polarized light that after being input to the VOA is transmitted by a half-wave plate, becoming TM polarized light and the other component is transmitted through the VOA after being transmitted by a half-wave plate and becoming TE polarized light from TM polarized light. Subsequently, the TM polarized light and TE polarized light are coupled by the circulator and output. At the time of transmission through the VOA, each of the split components of the input light is a TE wave and therefore, PDL does not occur. Nonetheless, with this configuration, since the split components of the input light are again coupled and output, this configuration is not applicable to schemes used on the respective output of the two split DP-QPSK light waves.